An air brake actuator assembly is often used in a wheel of a vehicle, especially in the wheel of a heavier vehicle, such as a truck or bus. The air brake actuator assembly usually includes a cast iron brake housing with an air chamber. The air chamber is typically made of mild steel (i.e., low-carbon steel) and comprises a front shell and a rear shell that are attached to each another to form an airtight seal. The front shell is attached to a flange of a brake housing, typically by screws, to form a weather-tight seal. The weather-tight seal is necessary to prevent water or other foreign matter, such as brake dust or dirt, from entering the air chamber. The air chamber contains a piston connected to a brake actuator by a suitable linkage.
Pressurized air is introduced behind a diaphragm of the piston, which generates a load in a push rod. The push rod in turn applies a force to a pivotally mounted operating shaft, causing it to rotate. An amplified force is applied to one or more adjustable tappet assemblies via eccentric action. This amplified force advances a brake pad toward a brake rotor. The brake pad causes the rotor to restrict rotation due to friction generated through contact between the brake pad and the tappet and through contact with a further brake pad mounted on the opposite face of the rotor.
WO-A-98026968 discloses a brake assembly comprising a cast brake housing that incorporates a cast air chamber. The air chamber is closed by a lid, which is made of mild steel. The lid and the air chamber are attached to each another using a suitable fastening means to form an airtight seal therebetween.
Known brake assemblies having a separate air chamber screwed to the housing experience a number of disadvantages. For example, screwing the front shell to the flange of the brake housing requires additional machining, parts and labor, thereby increasing the total cost of the finished brake assembly. Moreover, the weather-tight seal between the brake housing and the front shell introduces further complexity into the assembly. The flange adds mass to the brake assembly and reduces the available space around the brake assembly, thereby limiting design flexibility for other components near the brake assembly.
The cast brake housing shown in WO-A-98026968 overcomes some of these problems, but it introduces its own problems by increasing the mass of the brake assembly. This is undesirable because the brake assembly is an unsprung mass, which should be minimized to achieve optimum response of the vehicle suspension. The wall thickness of the cast air chamber could be reduced to reduce the mass of the brake assembly; however, this would not be desirable because castings do not perform well in tension. Thus, reducing the wall thickness would reduce the resistance of the cast air chamber to impact damage. The requirement for the mild steel lid to close the cast air chamber requires a different joining method, which may add further cost to the finished brake assembly.
EP0510930 shows a hydraulic brake caliper having a prefabricated cast-in-place cylinder bore liner. The liner is made from steel and the caliper body is made from aluminum. However, the problem being addressed here is to provide a lightweight caliper that nevertheless has a cylinder bore liner with good machineability and wear properties. This is in contrast to one of the problems addressed by the present invention, which is to provide an alternative attachment method of an air chamber component to a brake housing.